Several types of railroad cars are used to haul bulk shipments of materials. Fluids such as fossil fuels and various liquid chemicals are typically carried in entirely enclosed liquid tanker cars. Tanker cars are generally filled pumping the liquid into the top of the tanker through one or more closable openings. The openings are capped to protect the contents of the tanker from the elements and to prevent foreign substances from entering the tanker. Such tanker cars are generally unloaded through several valves carried by the lower portion of the tanker. The force of gravity acting on the liquid forces the liquid out of the tanker and into a receptacle.
Particulate or granular commodities such as coal and grain are generally transported by rail in open-topped hopper cars. Such hopper cars generally have a number of vertical walls extending upwardly from a floor comprised of a plurality of conical or "V" shaped depressions that converge downwardly into a flow control valve system carried by the lower portion of the car. The hopper cars are loaded through the upper open top and are unloaded through the valve controlled openings carried by the lower portion of the car.
By their very construction tanker cars are unsuitable for hauling anything but fluids. Similarly hopper cars are unsuitable for hauling anything but granular commodities. A result of this dedicated use construction is that such cars frequently carry loads only in one direction, returning empty. For example, coal cars carry coal from mines to market, and return empty, reslting in high operating expenses for the railroad (including fuel for transporting empty cars) and high capital costs (for single purpose cars which ride the rails empty half the time.)